New Zealand's South Island is due for a major earthquake in the next 2 decades, according to a new approach for ferreting out ancient temblors and perhaps predicting new ones. The technique, described in this month's GSA Bulletin, uses the growth of lichens--symbiotic communities of algae and fungi--to find and date when earthquakes tumbled precariously poised rocks. The lichen clocks have uncovered several previously unknown quakes, and experts say they could be used to improve predictions of future ones.

A common method for dating quakes that mar Earth's surface is to cut into a fault line and measure the ratio of carbon isotopes in the buried peat or charcoal. Realizing that the dig-and-date approach often misses earthquakes that never broke the surface, geologists William Bull of the University of Arizona, Tucson, and Mark Brandon of Yale University sought other clues to buried quakes. They homed in on lichens, which spread about 15 millimeters every century on exposed rock surfaces. By measuring the width of lichens on rockfalls, Bull reasoned, he could tell how long the lichens had grown--and thus how long the rocks had lain in their current positions. From this he presumed he could tell when an earthquake or some other event had dislodged them. Such measurements, called lichenometry, had been used to peg floods to within a few years, but experts say it had never been used credibly with earthquakes.

To test the clock's accuracy, Bull used digital calipers to precisely measure the widths of 34,000 lichens at 90 mountain sites on the South Island. Using lichen growth on tombstones to calibrate their clock, Bull and Brandon were able to correlate growth on rockfalls to within about 200 kilometers of the epicenters of 11 earthquakes that occurred in the past 150 years. "It worked beautifully," says Bull, nailing most to within a decade or so--a significant improvement over the 40-year accuracy of carbon isotope dating.

And the technique is taking earthquake researchers into uncharted territory, too. The duo also found evidence of rockfalls caused by previously unknown major quakes that have shaken the island every 261 years or so for the last millennium. The next big one, Bull says, is due sometime in the next 2 decades. The results are "astounding," says geologist Doug Burbank of Pennsylvania State University in University Park. Before this, he says "no one had been clever enough to realize you could find records of earthquakes so far away from the fault line." But not everyone is so convinced: "Boulders fall for lots of reasons," says California Institute of Technology geologist Kerry Sieh. However, he says, if the dates of major quakes in New Zealand's recent past are correct, "it would be hard for anyone to say [Bull and Brandon's New Zealand] forecast is wrong."